Apnea of prematurity is a major clinical problem whose pathophysiology is poorly understood. In preterm infants, especially those with apnea, ventilatory responses to hypercapnia are impaired: preliminary data reveal that hypercapnia causes prolongation of expiratory duration, similar to that observed during hypoxic exposure in early postnatal life. Hypercapnia induced prolongation of expiration is reversed by blockade of GABAA receptors in our neonatal animal models, while hypoxia induced frequency decrease is attenuated alpha2 by adrenergic receptor antagonists applied to the brainstem. We have demonstrated that catecholamine containing cells innervate bulbospinal neurons in newborn rat brainstem and that a subpopulation of bulbospinal inspiratory neurons express GABAA receptors. This proposal tests the hypothesis that hypercapnia modulates respiratory timing and the balance of inspiratory and expiratory muscle behavior during early postnatal development. We further hypothesize that C02 induced activation of central inhibitory systems such as GABAergic and alpha2 adrenergic pathways is responsible for maturational changes in these responses. We will administer intracisternal GABAA and alpha2 adrenergic blockers to characterize the role of neuroinhibitory pathways in C02 induced expiratory prolongation in maturing rat pups. We will utilize newborn piglets to characterize the role Of C02 induced chest wall and laryngeal muscle responses in modulating laryngeal resistance during changes in respiratory timing. Microinjection of GABAA and alpha2 adrenergic blockers at sites where brainstem bulbospinal neurons are identified will allow us to characterize the role of these neuroinhibitory pathways in mediating respiratory timing in piglets. Transneural labeling techniques will be combined with immunohistochemistry to characterize the connectivity of neurons in the medulla of maturing rat pups. We will describe expression and agonist affinity of GABAA and alpha2 adrenergic receptors on brainstem inspiratory bulbospinal neurons to test the hypothesis that receptor overexpression or increased agonist affinity contributes to the prolongation of expiration induced by hypercapnia during early life. Characterization of central neurochemical pathways that contribute to impaired hypercapnic ventilatory responses may advance our understanding of the pathogenesis of neonatal apnea and contribute to novel pharmacologic interventions.